There are a number of factors influencing how much UV we are exposed to as we are out and about, including altitude. There are a variety of reasons for this, including how the UV rays are absorbed by the ozone layer and amount of cloud cover. Please note the explanation that you are simply closer to the sun is not one of the reasons why altitude affects UV!
Before any further discussion, let’s remember that UV-B rays are the primary focus when thinking about the affects of altitude. UV-C rays are filtered out entirely by the ozone layer, while UV-A rays are basically unfiltered. Thus, it is the UV-B levels we care about here.
The majority of UV-B rays are in fact absorbed by the ozone layer. Most of the ozone layer subsides in the lower part of the stratosphere at around 20-30km above sea level. As a result, altitudes greater than 20-30km see much greater levels of UV-B. In fact UV-B levels are up to 350 million times higher at the top of the atmosphere than at the earth’s surface. These heights are greater than most people will ever go in their lifetime (Mount Everest stands at 7,200m above sea level and planes reach a maximum of 10,000-13,000m).
Below 20-30km, the general rule is that UV-B levels increase by 4% every 300m gain in altitude. A thinner atmosphere is the primary reason for this, as there are fewer particles to absorb UV. At low altitudes, the presence of tropospheric ozone (smog), aerosols, and other substances such as dust reduce the UV-B levels that reach the ground. These chemicals also become less dense as altitude increases.
Another side effect of high altitudes is changes in cloud cover. Clouds have the ability to reflect and scatter UV-B radiation. Typically clouds form anywhere between ground level and 18,000m high, with the cloud’s depth depending on its type. In general, the higher the altitude above ground level, the decreased likelihood of UV absorption by cloud cover. Mountains are an exception to this in that they tend to catalyze cloud formation above them for reasons relating to airflow and pressure. However, due to reflection from snowfall and a thinner atmosphere as mentioned earlier, mountaintops still tend to show much higher UV-B levels.
The bottom line here – altitude creates greater risk of skin damage secondary to higher UV-B levels. Be extra sun safe at high altitude!